In their most simple form, the inner and outer urfaces of these tubes are smooth, and they may easily be continuously manufactured by extrusion with the aid of a tubular die assembly, i.e. a die assembly formed by an outer die and an inner punch; it then suffices to ensure at one of the ends of sections of such tubes the swell adapted to allow their successive fitting.
In practice, it is usual to place the tubes of this type directly in the ground, this resulting in their undergoing transverse external forces from the latter which firstly tend to ovalise the section thereof and then, if these forces become sufficiently great, to provoke a buckling of this section then a sharp, local subsidence thereof.
To overcome this drawback, it has already been proposed to increase the moment of inertia per unit of length of these tubes by giving a finned profile longitudinally with respect to their outer surface.
French patent application No. 73 24218 (published under No. 2 237 107) describes an apparatus for continuously manufacturing, by extrusion, a tubular section made of synthetic material having a smooth inner surface and an outer surface provided with roughness of any configuration, for example annular fins as illustrated in FIG. 4.
This apparatus comprises a tubular extrusion die assembly formed by an outer die and an inner punch, in combination with, on the one hand, two sets of chills each presenting a casting impression provided transversely with grooves of circular transverse section alternating with ribs, likewise of circular section, thus forming the desired fins, and, for each of said sets of chills, conveyor means for a continuous circulation thereof along a closed loop path comprising, in particular, a rectilinear forward branch, which extends parallel to the axis of the die assembly, on either side of the outlet thereof, on one side of this axis for one of said sets of chills and on the other side of this axis for the other set, and in the course of which, from one of said sets to the other, the chills are paired so as jointly to form successive casting rings and, on the other hand, in line with the punch of the die assembly, a smoothing mandrel, said smoothing mandrel and said chills jointly defining the casting cavity for forming the desired tubular section.
As results from FIG. 4 of the above-mentioned French patent application, the ribs of the chills of this apparatus, which alternate with grooves, have, like the latter, an overall circular transverse section, such a section extending in both cases over a semi-circonference.
To all appearances, the radius of the transverse section of such a groove is equal to that of the transverse section of a rib.